1. Field of the Invention
The present invention is directed to methods for estimating corrections for the image degradation produced in medical ultrasound images by pulse reverberations and phasefront aberrations of the ultrasound wave in the tissue. The method hence has applications to all situations were ultrasound imaging is used in medicine, and also other similar situations of ultrasound imaging.
2. Description of the Related Art
Spatial variations in the acoustic properties of tissues (mass density and compressibility) are the basis for ultrasound back scatter imaging of soft tissues. However, with large variations of the acoustic properties in complex structures of tissue, the following effects will degrade the images:                i) Interfaces between materials with large differences in acoustic properties can give so strong reflections of the ultrasound pulse that multiple reflections get large amplitudes. Such multiple reflections are termed pulse reverberations, and add a tail to the propagating ultrasound pulse, which shows as noise in the ultrasound image.*        ii) Variations of the acoustic velocity within the complex tissue structures produce forward propagation aberrations of the acoustic wavefront, destroying the focusing of the beam mainlobe and increasing the beam sidelobes.*        
The reduced focusing of the beam main lobe reduces the spatial resolution in the ultrasound imaging system. The pulse reverberations and the increase in beam side lobes by the phase front aberrations, introduce additive noise in the image, which reduces the ratio of the strongest to the weakest scatterer that can be detected in the neighborhood of each other, defined as the contrast resolution in the image. This noise is termed acoustic noise as it is produced by the transmitted ultrasound pulse itself. Increasing the transmitted pulse power will hence not improve the power ratio of the signal to the noise of this type, contrary to what is found with electronic receiver noise. It is therefore a challenge to reduce the image degrading effect of pulse reverberations and phase front aberrations in the body wall in many applications of medical ultrasound imaging.
The materials with largest differences in acoustic properties are muscles, fat, connective tissue, cartilage, bone, air, and the ultrasound transducer itself. The body wall often contains mixtures of such tissues, and body wall phase aberrations and pulse reverberations are therefore a major cause of image degradation found with non-invasive ultrasound imaging in many patients. Especially, due to the large reflection coefficient of the transducer array itself, pulse reverberations that involves at least one reflection from the transducer array often produce disturbing noise in the image.
With a two-dimensional transducer array, the effect of the phasefront aberrations can in many situations be reduced by adding corrective delays and gain factors to the signals for the individual array elements, in the following referred to as element signals. Such correction schemes have been presented by many researchers. In more complex situations of tissue mixtures, the phasefront aberrations can produce modifications of the pulse-form, that can be corrected by a filter for each of the element signals. Such correction filters give the most general correction method, and delay/amplitude corrections can be considered as a special case or an approximation of correction filters.
However, it is generally a problem to estimate the correction filters from the signals that is back scattered in typical imaging situations, as for example described in [2]. One major problem for such filter estimation is the noise produced by pulse reverberations in the body wall, especially the strong pulse reverberations that includes reflections from the ultrasound transducer itself. This invention presents new methods for reduction of the noise from pulse reverberations. This noise reduction is then used in conjunction with methods for estimation of correction filters, or the approximate amplitude and delay corrections, for the phase front aberrations of the forward propagating wave.
For better understanding of the aspects of the invention, we shall give a classification of the multiple scattering of the ultrasound in the tissue (pulse reverberations). The forward scattering is included in the spatially variable propagation velocity, and hence is included in the phase front aberrations. For visible noise from pulse reverberations, the first scatterer must be within the transmit beam, and the last scatterer must be within the receive beam. This requires in practice an odd number of scatterings for the reverberations to be visible, and as the pulse amplitude decreases for each scattering, it means that third order scattering is the major component of reverberation noise in the image. Such third order scattering is conveniently grouped into five classes, where the three first classes includes at least one reflection from the transducer array surface:
Class I: This class includes a first scattering from a first structure in front of the object (like in the body wall), followed by a reflection of this back scattered signal from the transducer array, followed by at least a third scattering from a second structure in front of the object. We call the range of these objects for Zone I with reference to FIG. 4 below.
Class II: This class includes a first scattering from a first structure in Zone I in front of the object (like in the body wall), followed by a reflection of this back scattered signal from the transducer array, followed by at least a third scattering from a second structure inside the object. We call the range of these second objects for Zone II with reference to FIG. 4 below.
Class III: This class includes a first scattering from a first target inside the object (Zone II of FIG. 4), followed by a reflection of this back scattered signal from the transducer array, followed by at least a third scattering from a second structure in front of the object (Zone i).
Class IV: This class includes a first scattering from a first stationary structure, followed by a second scattering from a second stationary tissue structure, followed by at least a third scattering from a third stationary structure.
Class V: All other types of pulse reverberations that are not described by Class I-IV.
We note that Class IV can include Class I-III reverberations, as the essential point about Class IV is that all scatterers are stationary, which allows special processing to reduce the reverberations as discussed below. 2nd harmonic ultrasound imaging reduces Class I and Class II reverberations, because the 2nd harmonic components in the outbound pulse is low as the wave passes the body wall, and the amplitude of the first scattering in Class I and Class II is very low. 2nd harmonic components in the outbound pulse increases with propagation distance because the positive pressure swing in the pulse propagates with higher velocity than the negative pressure swing, due to the non-linear elasticity of the tissue. At the object depth, the 2nd harmonic components in the outbound pulse has increased to sufficient energy for imaging, and the Class I and Class II reverberations are greatly reduced, which has introduced a wide spread use of the method.
However, some 2nd harmonic components of the outbound pulse also exists within the body wall, and further reduction of the reverberation noise is in many situations necessary for adequate estimation of the phase aberration correction filters. Methods presented in this patent can be used for such reductions in the pulse reverberations, also in conjunction with other methods such as 2nd harmonic imaging.